Chapter 2 Selection of Uplink Antenna
The primary restrictions in the design for every spacecraft subsystem have been due to the size of the spacecraft and the stack configuration required for part of the mission. The top and bottom of the structure is a hexagon with a major diameter of 46.4 cm (18.25 in). The height of the structure is 30.5 cm (12 in). The limited available surface area of the spacecraft has been a persistent challenge in the layout of the solar panels to satisfy power requirements.
Another physical limitation considered in the selection of the uplink receive antenna is the maximum allowable clearance between the spacecraft while they are in the stack configuration. The satellites will be placed in preliminary orbits by the Space Shuttle. The three satellites will be stowed in the cargo bay during the shuttle flight and will be initially deployed in the stack configuration.
The satellites will be inter-connected by LightBands™. The LightBand will connect the top of one spacecraft to the bottom of the next. A LightBand will limit the maximum clearance between two satellites to be less than 5 cm (2 in). Once the LightBands detach, each of the three satellites will then use propulsion to reach their respective orbits. Soon after the satellites detach, both the uplink and downlink will be tested to establish contact with one of the groundstations.
The uplink signal transmitted from the groundstation will be circularly polarized. Circular Polarization (CP) is generated by exciting two equal-amplitude, orthogonal modes in phase quadrature. The quality of polarization in circular systems is linked to how the orthogonal modes in the antenna are excited and how well they can be controlled .
All radiated waves are elliptically polarized which can be defined by three variables: Axial Ratio, tilt angle, and sense. Linear and Circular Polarization are special cases of elliptical polarization. CP is more difficult to produce than linear polarization (LP), and CP antennas are generally more complex than LP antennas .
Axial Ratio is the ratio of the maximum to the minimum orthogonal components of the radiated E field. A perfectly circular polarized wave will produce a theoretical Axial Ratio of unity. The components are ninety degrees out of phase and the sign of the relative phase between them determines the sense of polarization .
For a given antenna, the quality of CP is specified by the Axial Ratio, and the tilt angle depends on the rotational orientation of the antenna . Ideally, a CP antenna will have its direction of maximum gain aligned with the direction of its best Axial Ratio.
Circular Polarization may be achieved with several different antennas. Two methods are used to generate circular polarization. Type 1 antennas produce CP due to their unique physical geometry. Examples include helix and spiral antennas. The sense of polarization is determined by the sense of the winding. Type 2 CP antennas contain hardware to explicitly generate spatially orthogonal components in phase quadrature .
Figures 2.1 (a) and (b) illustrate two antennas which were initially considered for the project. The microstrip patch and the Quadrifilar Helix are both circularly polarized and both have been used on previous space missions.
The microstrip patch antenna is very popular for its low-profile, and is used extensively for cellular and GPS links. Microstrip patch antennas are typically used at frequencies above 1 GHz . The microstrip patch falls into the class of resonant antennas where the operating frequency of the antenna depends on the physical -dimensions.
A common method to excite circular polarization in a microstrip antenna is the degenerate mode patch fed by a single line. The antenna requires minimal space for the feed network, is compact, and has been adopted for many practical antennas .
Microstrip patch antennas have been designed to receive the circularly polarized GPS L1 frequency of 1575.42 MHz which corresponds to a free-space wavelength of lGPS=19.0cm. A circularly polarized rectangular patch antenna made from Duroid with a dielectric constant,
- R=2.2, would require a side length ,
- (0.49)•(19.0cm)•(1/ Ö2.2)
- 28 cm (2.47 in)
However, a CP patch antenna designed to operate at the uplink frequency of 450 MHz, would be nearly 3.5 times larger. Using a free-space wavelength of 67 cm and the dielectric constant for Duroid, the side length would need to be 22 cm (8.7 in). The patch antenna would require a minimum surface area of 484 cm2 (75.0 in2).
The total area available on the bottom surface of the spacecraft (including the “stay out” zones due to the Lightbands) is nearly 1360 cm2. A significant percentage of the bottom surface area has been allocated for solar panels. The shortage of available surface area for mission critical solar panels precluded the possibility of choosing an uplink patch antenna that would require over 1/3 of the available surface area of the spacecraft.
Material with a higher dielectric constant may be used to reduce the surface area required for an uplink patch antenna. Several high dielectric materials (eR»10) are commercially available. Ceramic substrates are produced by Coors Porcelain Company (ADS-995), 3M Technical Ceramics Division (AlsiMag 838), and the Materials Research Corporation (Superstrate 996), with relative permittivities of 10.1, 10.0, and 9.9 respectively .
Using a nominal relative permittivity of 10.0, the side length will need to be 10.6 cm (4.2 in). The required surface area for the high permittivity substrate patch would be approximately 112 cm2 (17.4 in2).
However, the typical microstrip patch antenna is extremely narrowband below 1.0 GHz The impedance bandwidth (VSWR<2.0) for a microstrip patch antenna is a function of the patch geometry and the relative permittivity of the substrate (eR) .
In terms of fabrication, a microstrip patch antenna with an impedance bandwidth less than one percent is impractical. The accuracy required to achieve radiation at the desired frequency or even in the correct bandwidth is extremely difficult to obtain. Also, microstrip patch antennas are not conducive to tuning after fabrication.
Some techniques to improve bandwidth include using thicker substrates, using a low dielectric material (eR»1), or using a matching structure . Neither the matching structure nor the low permittivity substrate is viable due to the resulting increases in required surface area. Solving the required substrate thickness for a one percent impedance bandwidth yields an electrical thickness (t/l) = 0.0295. At the UHF operating frequency of 450 MHz, the actual thickness of the substrate would need to be 1.96 cm (0.774 in).
High quality, high permittivity substrate materials are not readily available on the order of the thickness required to create a UHF patch antenna. For example, of the three materials mentioned, AlsiMag 838 is available with a maximum thickness of 2.0mm (0.08 in) .
Microstrip antennas with thick substrates can also excite surface waves which propagate along the air-dielectric interface. The presence of surface waves can produce undesirable effects on the radiation patterns, and can reduce the radiation efficiency and bandwidth of the antenna .
Bandwidth enhancement can be achieved by increasing the effective volume of the patch antenna and introducing parasitic elements. The technique of stacking patches, horizontally or vertically, is another method to achieve the wideband characteristic desired in practice . The design of a stacked patch antenna requires the use of finite-difference time domain (FDTD) computation software. There is also minimal tuning capability once the antenna is fabricated. And finally, the coupling which results between the different patches results in a significant loss in efficiency.
The physical requirements for a microstrip patch antenna to operate at the UHF frequency band specified by the ION-F project, precluded its use on the satellite uplink.
A second option for the on-board uplink receive antenna would be a Quadrifilar Helix (QFH). The QFH has very good CP properties, and the endfire radiation pattern would be ideal for a satellite application .
The QFH has an established space heritage and details of the design are available .
However, the minimum dimensions for a QFH at a frequency of 450 MHz preclude the possibility of its use for this application. The maximum allowable clearance on the zenith and nadir surfaces of a ION-F nanosatellite separated by one LightBandT M is less than 5 cm. Using the design values for the Air Force 5D and the AMSAT OSCAR 7 quadrifilars, the axial length of a QFH at the uplink frequency of 450 MHz would need to be, as follows:
- 27 • (66.7 cm)
- 0 cm (7.1 in)
which is several times larger than the maximum clearance permitted.
The design of an uplink receive antenna to operate at UHF with circular polarization and with the physical restrictions of the ION-F spacecraft was at an impasse. A design tradeoff study concluded the solution was to investigate low profile, limited surface area antennas that are not circularly polarized. A linearly polarized antenna receiving a circularly polarized transmitted signal will suffer a theoretical loss of 3 dB . However, the reduction in received power due to the polarization loss may be offset by the increased gain of the proposed receive antenna. Other parameters in the link budget (e.g. transmitted power) can be varied to compensate for the polarization mismatch at the uplink receive antenna.
The possible use of the bottom surface of the spacecraft was also considered in the selection process. The nadir surface could act as a ground plane, which would improve the total gain patterns of the receive antenna.
A one-wavelength resonant loop antenna mounted above a ground plane was considered for this application. The resonant loop antenna has a low profile, requires limited surface area, and can be designed for a moderate gain of approximately 9 dB. Richtscheid and King have analyzed the radiation properties of one-wavelength circular  and square  loop antennas in free space. The characteristics of a resonant loop above a planar surface have also been investigated . A hexagonal shape was considered to facilitate the mechanical connection to the isogrid pattern of the spacecraft.
The remaining chapters of this paper will verify the similarity of the hexagonal loop to the well-known circular and square resonant loop antennas. Once the similarity between the hexagonal loop to the circular and square loop antennas is established, published circular and square loop data will be used to determine the preliminary design parameters for a one-wavelength hexagonal loop antenna mounted above a ground plane. Calculations using more exact numerical methods will then be used to finalize the geometry of the prototypes.
Methods to optimize the interface between the coaxial feed and the antenna input terminals are also investigated. A network at the antenna input terminals composed of a gamma match is used to maximize the received power. A sleeve balun transformer is integrated into the feed to isolate the antenna from the transmission line.
Prototypes of a resonant hexagonal loop antenna with the matching network are fabricated, and measurements are evaluated to investigate the feasibility of the design for the ION-F mission.
Chapter 1: Introduction
1.1 Project Background
1.2 Nanosatellite Overview
1.3 Overview of the Uplink
Chapter 2: Selection of Antenna
2.1 Microstrip Patch
2.2 Quadrifilar Helix
2.3 Design Tradeoff
Chapter 3: One Wavelength Resonant Loop Antenna
3.1 Resonant Loop Antenna in free space
3.2 Resonant Loop Antenna over a Ground Plane
3.3 Gamma Match
3.4 Balun transformer
Chapter 4: Fabrication and Testing of Prototypes
4.1 Prototype Fabrication
4.3 Far field Pattern Measurements
Chapter 5: Conclusions
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Electrical Design and Testing of an Uplink Antenna for Nanosatellite Applications